The invention relates to heat exchangers of the tubular type, such as, for instance, feedwater preheaters, condensers and steam generators.
A problem involved with heat exchangers of this type arises due to the fact that the tubes adopt severe oscillations caused by turbulence and instability of the flow of liquid around the tubes. At times, the oscillations are so intense that the tube material is rapidly fatigued, a situation which often arises with condensers, for instance. It may even happen that the tube "beats" within a clearing between the tube and tube support plates provided with apertures through which the tubes extend, resulting in an abrasion of the tube material at contact surfaces between the tube and the support plate. The wear may proceed to such a degree that severe leaks arise. Evidently, such leakages are impermissible in nuclear reactor plants.
The principal object of the present invention is to provide a feasible means for solving this problem and particularly so in steam generator plants in which tube wear has already been observed or in new plants in which such wear may be expected.
With this object in view, the present invention resides in a device for providing a substantially uniform and vortex-free inflow and distribution of feedwater to a heat exchanger constituting a steam generator and comprising a plurality of tubes constituting a tube bundle for a primary fluid to heat said feedwater as the secondary fluid, and a generator shell enclosing the tube bundle and having an inlet nozzle, said inlet nozzle having therein a diffuser structure characterized by a number of diffuser channels adapted to restrict an outflow of water from the generator shell through said inlet nozzle during a break in a feedwater pipe connected to said inlet nozzle and arranged within the inlet nozzle and by baffle means associated with said diffuser structure to deflect the feedwater flow in a radial direction about said inlet nozzle and arranged closely adjacent the inlet nozzle between the downstream ends of the diffuser channels and the tube bundle enclosed by the shell.
The situation involved with heat exchanger tube oscillations is, generally, similar to the action of wind on non-stayed funnels. Inwardly directed vortexes, so-called Carman vortexes, arise in the windshadow behind the funnel, such vortexes giving rise to pulsating lateral forces. Such pulsating forces adopt a frequency f.sub.c which is dependent on the funnel diameter D and the wind velocity U. When, for a stayed funnel with low damping, the resonance frequency of the funnel is within the range f.sub.c =0.2 to 0.7 times U/D, there is a risk that the oscillations will adopt such amplitudes that the funnel is damaged. The coefficient 0.2-0.7 is the dimensionless Strouhals number S. EQU S=f.sub.c .multidot.(D/U)
Extensive investigations on tube heat exchangers have shown that for high fluid velocities outside and crosswise through a tube bundle having one or more rows of tubes, severe vibrations arise for Strouhal's number within the range S&gt;2, based on the velocity within the cross section between the tubes, or, for a normal tube pitch S&gt;0.7, the entire free area in front of the tube bundle taken as a basis, substantially in analogy with the situation with unstayed funnels. If, in addition thereto, the flow is pulsating as to direction and intensity, the risk for tube oscillation is markedly increased. Usually, the oscillations show nodes at the support plates. Higher frequencies with nodes between the support plates may as well be present.
In general a flow distributor for a shell and tube heat exchanger having a shell side inlet nozzle so disposed in the shell that the central axis thereof is generally perpendicularly oriented with respect to the tubes, when made in accordance with this invention, comprises a flow distributor disposed within the shell on the shell end of the inlet nozzle. The flow distributor comprises a plurality of vanes which generally direct the flow of fluid from the inlet nozzle into the shell in a generally radial direction with respect to the axis of the inlet nozzle and a plurality of converging and diverging venturies disposed within the inlet nozzle upstream of the flow distributor vanes.